The present invention relates to the charging of electrochemical capacitors designed for the storage of electrical energy. More particularly, the present invention is directed to providing an effective method for charging electrochemical capacitors when the charge power changes over time, and to ensuring the timely completion of the charging process once an optimal state of charge is reached.
Heterogeneous electrochemical supercapacitor (HES) energy storage devices have been increasingly used for electrical energy storage (see, e.g., U.S. Pat. No. 6,518,734 to A. Nourai, et al., entitled “System And Method Of Forming Capacitor-Based Electrical Energy Storage Modules”). Traditionally, electrical energy storage by rechargeable power sources has been dominated by storage batteries. However, various features of HES devices make their use for electrical energy storage superior to batteries. For example, while HES devices may be inferior to batteries in terms of the value of their specific weight and volume of stored energy, HES devices are significantly better than batteries in terms of life cycle, range of allowable values of charge current, and rapid charging capability. As such, HES devices are well-suited to use as, without limitation, energy-intensive power sources for industry, telecommunications systems, public utility companies, electric vehicles and hybrid vehicles; as electrical energy storage devices for electric power generated in stationary or portable (transported or field) wind and hydroelectric installations; and as portable power sources, charged from muscle driven generators, etc.
In order to effectively use any sealed electrical energy storage device, it is necessary to provide for control of its charge process. The list of the parameters monitored and controlled during the charge process depends considerably on the parameters of the electrochemical system, its particular design, and the field of use of the energy storage device.
There are a great many known methods for controlling the charging process of batteries. For example, various charging techniques exist in which the state of charge is determined and charge control is performed by controlling changes of the main parameters of the battery over time via controlled changes of the charge mode. In one such process, measurements of current and voltage during the charge-discharge cycle are taken and the measured parameters are compared with pre-assigned values, which may be determined experimentally or by mathematical modeling. Controlled changing of the charge mode is then performed based on said comparison. In another such process, the first or second time derivative of voltage is measured, temperature changes during the course of the charge process are controlled, and changes in the current or voltage in different steps of the charge process are controlled. In yet another such process, the charge voltage or current is controlled by changing the parameters of the outgoing charge current pulses or pulse trains.
There are, however, fundamental differences between batteries and electrochemical capacitor energy storage devices that prevent or teach against the use of identical charging process control schemes. Primarily, in electrochemical capacitors, unlike batteries, it is the value of the voltage to which the capacitor is charged that determines the value of the electrical energy that can be stored thereby. Thus, a desired value of stored electrical energy is obtained by charging such a capacitor to a specified charge voltage value, which charge voltage value is chosen from the operating voltage range of the capacitor. When choosing this specified charge voltage value, it is necessary to take into account the current temperature condition of the capacitor, which typically changes rapidly during a high-current charging process.
Therefore, other known capacitor charging processes have been developed. For example, it is known that capacitors may be charged by constant current, constant voltage, or constant power techniques. Charging under constant current to the specified voltage is likely the most commonly used charging method.
As mentioned above, the value of the specified voltage depends on the operating temperature and the value of the charge current, and is set empirically for this design of a particular capacitor. For example, in patent application PCT/RU03/00109, WO03/081618 to 1. Varakin et al., entitled “Method Of Charge And Discharge Of Capacitor With Double Electric Layer”, the voltage at which the leakage current and power intensity of a capacitor are optimal at a given measured temperature of the electrolyte is assumed to be the maximum operating voltage of the charge.
When the specified value of voltage is reached, the charging of a capacitor may continue at constant voltage. As the state of charge increases, the value of the charge current decreases. Charging at constant voltage may be deemed complete, for example, upon expiration of a specified time or when the charge current decreases to a specified value.
One constant current charging method of interest is described in U.S. Pat. No. 5,640,080 (the '080 patent) to Mikitaka Tamai, entitled “Secondary Battery Charge”. The charging process described therein is performed by employing alternating periods of charging and rest. During each charging interval, the quantity of electricity ΔQ, which passes through the battery, is limited to a value that will not bring about any deterioration of battery parameters due to overcharging. This quantity of electricity is defined as a portion of the battery's full capacity (for example, not more than 5% of the rated capacity). Charging is deemed complete when the voltage measured across the terminals of the battery during a rest period becomes equal to or higher than a predetermined voltage value.
The batteries of the '080 patent are charged by alternating periods of charging (having their initial duration of Tcha) and rest (having constant duration Tre). Thereafter, the measured voltage Vch is compared during charging periods with the predetermined voltage value Vcha. When the measured voltage Vch across the terminals is equal to or exceeds the predetermined voltage value Vcha, the battery is deemed to be fully charged. Each time the measured voltage Vch reaches the predetermined value Vcha, the quantity of electricity provided in each subsequent charge period (which is followed by a period of rest) decreases. The charging process is complete when the quantity of electricity provided for charging reaches a zero value.
In a variation of the charging process, the '080 patent teaches that each time the measured voltage Vch reaches the predetermined value Vcha, the pre-assigned quantity of charging electricity may remain unchanged while the duration of the subsequent rest period is increased. In other words, the change in temperature of the battery is taken into account. The value Vcha may be reduced, subject to the temperature of the battery.
When a HES is charged, the voltage across its terminals Uch depends on the parameters of the HES and the conditions of the charge process:Uch=U0+IR+ΔU  (1)wherein U0 corresponds to the stationary voltage which is set in the charged capacitor after the completion of the charge and depolarization processes, IR corresponds to voltage surge when the charge current is turned on and off, and ΔU is explained in more detail below.
The value of the stationary voltage U0 is determined by the charge amount and capacitance of the capacitor C. Further, it is known that the capacitance of the capacitor is, in turn, a function of the potential of the polarizable electrode (φ), the state of its charge, and its temperature T (i.e., U0=Q/C(Q,φ,T)).
The voltage surge IR is a manifestation of the voltage drop at the Ohmic resistance. It is a function of the change in charge current amplitude over time I(t), and the internal resistance depends on the state of charge, potential of the polarizable electrode, and temperature of the capacitor (i.e., R(Q,φ,T)).
ΔU makes a considerable contribution to the change of voltage across the terminals of the HES during the charging process. One of the components in the value ΔU is determined by the polarization processes of the HES'. It depends on the values Q, φ, T, t, and also on the employed type of the electrochemical system and the particular design features of the HES. These design features may include, for example, conductivity, spatial structure and thickness of the polarizable and non-polarizable electrodes, conductivity of the electrolyte, and thickness of the separator.
The second significant component in the value ΔU is determined by the processes taking place during the gas cycle of the HES, which unfolds in systems that employ aqueous non-organic acid solutions as electrolytes. This second component is related to the processes of oxygen evolution in the positive electrode and of hydrogen in the negative electrode, when the values of the electrodes' potentials exceed equilibrium values. When a HES nears a maximum state of charge, the quantity of electricity contributing to charging decreases, and the quantity contributing to electrolyte decomposition increases. The voltage across the terminals of the HES then reaches the maximum value.
These processes also result in the reduction of evolved oxygen in the negative electrode, and the formation of water, which brings about depolarization of the potential of the negative electrode and, accordingly, a reduction in voltage of the HES. Exothermic reactions associated with the reduction of the oxygen are accompanied by an increase in HES temperature. As a result, the overpotential of oxygen and hydrogen evolution of the positive and negative electrodes, respectively, is reduced, and the voltage in the across the terminals of the HES begins to decrease. When charging current is further passed, an increase in the amount of oxygen evolved in the positive electrode and an increase of gassing inside the case of the HES also occurs.
Therefore, in the process of charging HES devices, the maximum voltage value is typically a changing parameter. Maximum voltage depends on specific electrochemical and design characteristics of the HES, as well as its state of charge, current temperature condition, and rate of charge. The value of the quantity of charge electricity, at which the maximum voltage is reached in these particular conditions, corresponds to the optimal state of charge of the HES and the level of its Coulomb and energy losses.
From the foregoing description, it would be apparent to one skilled in the art that the depolarization and electrochemical processes taking place during the gas cycle of the HES bring about a decrease of its Coulomb capacity, a decrease in the energy efficiency of its charge-discharge cycle, and a growth of its internal resistance. Furthermore, when uncontrolled charging is performed, some reduction of the life cycle of a HES is also possible.
In practice, it is necessary to ensure that the charging algorithm applied to a HES device makes it possible to complete an effective charge, even when the value of the charge power is changing in a wide range, and irrespective of the initial state of charge and thermal condition of the HES. For example, a considerable change in the thermal condition of a HES may occur when the charge rate of the HES is high. Inasmuch as a HES is very capable of charging by currents of different values, the duration of a charging process may vary from several minutes to several hours. The ability to control changes in the charging rate makes it possible to choose optimal power and price parameters of the charger, on the one hand, while imposing certain requirements on the versatility of the charge method, on the other hand.
Stringent requirements to the charging process algorithm are set for a floating charge (i.e., charge at constant voltage). For example, in order to compensate for capacity and energy losses that may occur when an electrical energy storage device is used as a back-up power source, or that may occur during long-term storage of such a device in its maximum charged condition, it is necessary to perform an additional charge. When the electrical energy storage device is a battery, this additional charging is performed by various methods. For example, additional charging may be performed at constant voltage, by currents of small value (that are close to the value of the leakage current), or by recurrent switching of the floating charge. These methods require an accurate setting of the floating voltage value, a limiting of the maximum charge current value, and the maintaining of stable temperature conditions. When the stationary thermal mode is violated, there is high risk of “thermal acceleration” during which the processes of the gas cycle may be accelerated, the temperature may go up, and the charge current may increase in a spontaneous and critically fast manner.
Currently, the modeling of the kinetics of the charge and discharge processes, subject to some of the aforementioned parameters, makes it possible (with a high rate of probability) to forecast energy characteristics of capacitors having different designs (see, e.g., D. Dunn, J. Newman, “Prediction Of Specific Energies And Specific Powers Of Double-Layer Capacitors Using A Simplified Model,” J. Electrochem. Soc., 147, 820 (2000); S. Kazaryan, S. Razumov, S. Litvinenko, G. Kharisov, and V. Kogan, “Mathematical Model Of Heterogeneous Electrochemical Capacitors And Calculation Of Their Parameters,” J. Electrochem. Soc., (2006), in press). At the same time, however, the practical task of controlling the charging process of HES devices remains unresolved.